A wireless data communication provides a reliable, robust, and efficient means for transmitting information. For example, radio frequency (RF) wireless communication is broadly used in wireless and cellular phones, wireless LAN, etc. However, RF systems have several shortcomings. One of the shortcomings of RF communication is associated with security of the transmitted information, due to the difficulties of controlling the direction of the radiation beam. Moreover, interference of radiation between various RF sources may occur. Likewise, the bandwidth and the number of channels available in a given region can be limited.
Another example of wireless data communication, such as Free Space Optical (FSO) communication can be less susceptible to these limitations. There are two basic FSO configurations according to which the FSO communication technique is based either on non-line-of-sight (diffuse) paths or line-of-sight paths between transmitter and receiver.
For example, diffuse Infrared (DIR) technology is known that enables the use of infrared optical emissions without the need for line-of-sight between the transmit communication node and receive communication node. It can create communication links at distances of over 10 meters (30 feet) or more, depending on the emitted optical power. Unlike a direct infrared signal, which emits light in a narrow beam, creating a line-of-sight, narrow angle communication link, a diffuse infrared device (usually Lambertian source) illuminates the room with an infrared signal, and then utilizes the reflections from the ceiling, walls, floors, and other natural surfaces to maintain robust optical communications.
An example of a Diffused Infra Red (DIR) technique has been proposed by InfraCom Ltd. (www.infra-com.com). This communication method uses ‘diffused’ reflections (scattering) of optical beams from ceilings, walls, floors, and other similar surfaces that scatter the light to link the transmitter and receiver. The receiver within the coverage space can detect the scattered radiation, which is modulated in order to provide information data transmission.
Non-line-of-sight communication systems are known which utilize modulated omnidirectional ultraviolet radiation in the solar-blind region of the electromagnetic spectrum. For example, U.S. Pat. No. 5,301,051 to Geller describes a system that enables omnidirectional non-line-of-sight simultaneous communications in a number of frequency separated channels in the ultraviolet spectrum. The system includes a plurality of discharge lamps each having at least a single different isotope of mercury for each omnidirectionally radiating a discrete hyperfine line in the ultraviolet spectrum. The hyperfine lines are composed in the 253.7 nm resonance line of a low pressure mercury discharge tube. Each of the discrete lines in the ultraviolet spectrum is modulated by an on-off sequence generator so that modulated discrete lines transmit communications to a number of receivers randomly disposed in a non-line-of-sight relationship to the discharge lamps. Simultaneous communications within the solar blind region are assured for non-line-of-sight transmitters and receivers within a limited range so that communications are not compromised.
Diffuse systems are robust to blocking and do not require that transmitter and receiver are aligned, as many paths exist from transmitter to receiver. However, multipath interference at the receiver can cause InterSymbol Interference (ISI) and the path loss for most systems is high. Thus, due to the high loss of diffused light, DIR is limited to tens of Mbps for the entire communication channel. Moreover, if several transmitters are used in the same room, they need to be time or frequency multiplexed and the total available bandwidth has to be split.
The alternative approach is based on Line of Sight (LOS) paths between transmitter and receiver. Usually, LOS technologies are divided into two groups, such as wide field of view (FOV) systems and narrow FOV systems. Wide FOV systems, for example, may use ceiling mounted transmitters that illuminate the coverage area. Examples of wide FOV techniques are technical solutions of the Infrared Data Association (IrDA) (see, for example, www.irda.org). These solutions are implemented in handsets, PDAs and laptop computers to provide a point-to-point IR non-contact communication link. The wide FOV communication is characterized by wide angle of reception and transmittance cones of 15-30 degrees. The wide FOV devices are relatively easy to use, as the user does not need to point the transmitter exactly at the receiver, and vice versa. The cost of the wide angle reception and transmission cones is limited range (of about few meters) and relatively low bandwidth. As the beams are narrowed, path loss reduces and the allowed bit rate increases, albeit at the cost of coverage.
An example of narrow FOV FSO technique is the FSO systems provided by fSONA Communications Corporation. These systems usually involve large and expensive telescopes that need to be precisely aimed at the narrow beam transmitter, and are capable to provide megabits/s to gigabits/s communication links over hundreds of meters to a few kilometers. In narrow FOV FSO communication systems the transmitter and receiver must be accurately directed at each other. Therefore, narrow beam systems either require tracking to allow user mobility, or some sort of cellular architecture to allow multiple narrow beams to be used.
A typical FSO transmitter includes one or more optical radiating elements (sources), and an optical module to shape the beam and render it eyesafe, if required. Usually, Light Emitting Diodes (LED) and/or lasers are employed as the optical radiating elements.
A typical single-element receiver includes an optical concentrator to collect and concentrate incoming radiation, an optical filter to reject ambient illumination, and a photodetector to convert radiation to photo-current. Moreover, amplification, filtering and data recovery are then required. In a single-element receiver, the desired signal, ambient light noise, co-channel interference, and (often undesired) delayed multipath signal are combined in a single electrical signal. The inherent trade-off between range and angle of reception can be resolved by using an angle-diversity receiver, which utilizes multiple receiving elements that are pointed in different directions. The multiple receiving elements comprise an array of detectors for receiving communication signals from optical radiating elements, thereby allowing the system to cover a wide angle of reception.
The angle-diversity receiver reads the communication signal with a narrow Instantaneous Field Of View (I-FOV) from each single detector element. The photo-currents received in the various elements are amplified separately, and the resulting electrical signals can be processed using various methods (see, for example, J. M. Kahn, etc., “Imaging Diversity Receivers for high-speed Infrared Wireless Communication”, IEEE Communications Magazine, December, 1998, PP. 88-94).
U.S. Pat. Publication No. 2003/020992 describes a free space optical communication network that includes plural stations, some capable of functioning as both transceivers and repeaters, and a station having such capability. The stations include a transmitter array having many optical emitter elements, each having an associated beam and a receiver having a receiver array with many optical detector element areas having beams corresponding with the beam of an emitter element of a transmitting optical station of the network. An optical arrangement associated with the arrays and the arrays themselves are such that beams associated with different elements of each array can be coupled with different stations of the network. The stations include one or more of the following features: (1) overlapping beams, (2) avalanche photodiodes in the receive array, (3) a filter arrangement for enabling only a desired wavelength to be transmitted from and received by the arrays, and (4) transmit and receive arrays at different locations in the stations so that photons emitted from the transmit array do not interfere with detectors of the receive array.